There are actually 2 ways to solve this, refer to the "Installation" guidelines: http://ankane.github.io/chartkick/
For Google Charts, use:
<%= javascript_include_tag "//www.google.com/jsapi", "chartkick" %>
If you prefer Highcharts, use:
<%= javascript_include_tag "path/to/highcharts.js", "chartkick" %>
For those who are thinking of using jsapi offline, you cant really, even if you have downloaded jsapi.js, it still does a further load to google. You are just breaking the terms:
Extract from: Google Charts FAQ
Can I use charts offline?
No; your computer must have live access to http://www.google.com/jsapi
in order to use charts. This is because the visualization libraries
that your page requires are loaded dynamically before you use them.
The code for loading the appropriate library is part of the included
jsapi script, and is called when you invoke the google.load() method.
Our terms of service do not allow you to download the google.load or
google.visualization code to use offline.
Can I download and host the chart code locally, or on an intranet?
Sorry; our terms of service do not allow you to download and save or
host the google.load or google.visualization code.